All the atoms in the reactants of a chemical reaction are reorganised to form all the products of the reaction.
There must be an equal number of each type of atom on both the reactant and product side of the equation.
Chemical formulae cannot be altered to balance a reaction equation; only coefficients can change.
Atoms cannot be created or destroyed. During a chemical reaction, atoms are simply rearranged into products.

Common misconception

Pupils sometimes think that chemical formulae can be changed to balance atoms.

Put boxes around each formula in the equation / pretend there is a bucket of pre-made molecules which they can use to balance the number of atoms on each side. Stress that it takes perseverance and practice before equations are balanced.

Keywords

Word equation - A word equation is a way of describing a reaction using the chemical names of the reactants and products.
Chemical formula - A chemical formula represents a substance using element symbols and the number / ratio of atoms of each element in the substance.
Symbol equation - A symbol equation is a way of describing a reaction using the chemical formulae of the reactants and products.
Balanced symbol equation - A balanced symbol equation describes a reaction using a symbol equation with coefficients, which ensure there are equal numbers of atoms of each element on both sides of the symbol equation.
Coefficient - A coefficient is the number placed in front of a chemical formula to balance an equation; it multiplies all the atoms in the formula and shows the ratio of substances in a reaction.

Use a 'box / bubble method' or use molymods / bricks to to balance equations. Pupils are only able to use what already exists in the reaction equation in order to balance it.

Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.

What elements are present in CaCO₃ and how many atoms are there of each?

calcium (1) and cobalt (3)

calcium (3), carbon (3) and oxygen (3)

carbon (1), actinium (1) and oxygen (3)

Correct answer: calcium (1), carbon (1) and oxygen (3)

Q2.

Match the substance names and symbol formulae.

Correct Answer:magnesium oxide,MgO

MgO

Correct Answer:aluminium iodide,AlI₃

AlI₃

Correct Answer:copper carbonate,CuCO₃

CuCO₃

Correct Answer:sodium hydroxide,NaOH

NaOH

Correct Answer:potassium sulfate,K₂SO₄

K₂SO₄

Correct Answer:strontium chloride,SrCl₂

SrCl₂

Q3.

Match the substance to the correct state symbol.

Correct Answer:water,H₂O (l)

H₂O (l)

Correct Answer:ice ,H₂O (s)

H₂O (s)

Correct Answer:steam,H₂O (g)

H₂O (g)

Correct Answer:dissolved sodium chloride ,NaCl (aq)

NaCl (aq)

Correct Answer:mercury at room temperature,Hg (l)

Hg (l)

Correct Answer:helium at room temperature,He (g)

He (g)

Q4.

The chemical formula for magnesium oxide is MgO. In a chemical symbol equation, how would you amend this to provide twice as many Mg and O particles?

Mg₂O₂

2Mg₂O

Correct answer: 2MgO

Q5.

When 12.2 g of iron reacted with an unknown mass of sulfur, 20.6 g of a product was formed. What was the mass of the sulfur?

Correct answer: 8.4 g

32.8 g

12.2 g

6.1 g

10.3 g

Q6.

Which symbol equation is correct for when bromine displaces iodine from sodium iodide solution?

Br (l) + NaI (aq) → NaBr (aq) + I (l)

Br₂ (g) + 2NaI (aq) → 2NaBr (aq) + I₂ (g)

Correct answer: Br₂ (aq) + 2NaI (aq) → 2NaBr (aq) + I₂ (aq)

Br₂ (l) + NaI (aq) → NaBr (aq) + I₂ (l)

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6 Questions

Q1.

What does a chemical formula like Na₂O represent?

chemical name of a substance

Correct answer: number and ratio of each element in a substance

physical state of a substance

Q2.

Match the terms to the correct definitions.

Correct Answer:word equation,describes a chemical reaction using chemical names

describes a chemical reaction using chemical names

Correct Answer:chemical formula,represents a substance using element symbols and numbers

represents a substance using element symbols and numbers

Correct Answer:symbol equation,describes chemical reactions using chemical formulae and state symbols

describes chemical reactions using chemical formulae and state symbols

Q3.

Starting with identifying the reactants and products chemical formulae, sort the steps to describe how to balance an equation.

1 - Identify the reactants' and products' chemical formulae.

2 - Count the initial particles of each type in the reactants and products.

3 - Add coefficients to each substance to add multiples, as required.

4 - Confirm the final number of particles in the reactants is equal to the products.

Q4.

A coefficient is the number placed in front of a chemical formula to balance an equation; it all the atoms in the formula and shows the ratio of substances in a reaction.

Correct Answer: multiplies, times

Q5.

Determine which reactions are balanced.

Correct answer: Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)

Correct answer: CaCO₃ (s) → CaO (s) + CO₂ (g)

Cl₂ (l) + KI (aq) → KCl (aq) + I₂ (l)

Correct answer: NaOH (aq) + HNO₃ (aq) → NaNO₃ (aq) + H₂O (l)

2K (s) + O₂ (g) → 2K₂O (s)

Q6.

Select the correct balanced equation for when propane (C₃H₈) completely combusts to form carbon dioxide and steam.

C₃H₈ (g) + O₂ (g) -> CO₂ (g) + H₂O (g)

C₃H₈ (g) + 2O₂ (g) -> 3CO₂ (g) + 4H₂O (g)

C₃H₈ (g) + 5O₂ (g) -> 3CO₂ (g) + 4H₂O (l)

2C₃H₈ (g) + 7O₂ (g) -> 6CO (g) + 8H₂O (g)

Correct answer: C₃H₈ (g) + 5O₂ (g) -> 3CO₂ (g) + 4H₂O (g)